Car::  Infiniti SUV

Was helping a friend hookup electrical lines for a restored trailer...

Had problems with trailer lights...and car lights would occassionally
go out from grounding issues with trailer...but after unplugging
trailer and turning ignition switch off, lights on car would work
fine.  

Almost as though a hear-induced breaker cooled and worked again.

We got to a point where rear running lights would not come on.  ALL
other lights are working fine.  It seems odd because brake lights,
directionals, hazard lights, front running lights, backups are all
still working fine.

It's just the rear side running lights as well as rear LED's for
running lights do not work.

We've checked all fuses inside the car (driver side kick panel) and
the inline fuses inside the battery enclosure.  All were intact.

So we're at a loss for understanding what is screwed up.

We have plans to bring the car into the dealership for a diagnostic
run, but I'm fishing for simple things we should check first.

Any ideas on how to remedy this problem would be greatly appreciated.

Thanks in advance.